The basic cooling mechanism of a fluorescent refrigerator requires a working material to absorb substantially monochromatic electromagnetic radiation at one frequency and then emit fluorescent radiation that has, on the average, a higher frequency. More energy is thereby removed from the working material than is introduced into the material, the difference between the output energy flux and the input energy flux being supplied by the thermal energy of the working material. Recent laboratory measurements have demonstrated laser-induced optical refrigeration in both solids and liquids. See, e.g., C. E. Mungan et al., Phys. Rev. Lett. 78, 1030 (1997) and J. L. Clark and G. Rumbles, Phys. Rev. Left. 76, 2037 (1996), respectively.
In U.S. Pat. No. 5,447,032 for "Fluorescent Refrigeration" which issued to Richard I. Epstein et al. on Sep. 5, 1995, one embodiment of an optical refrigerator is described. Therein, the working material is a cylinder with two opposing faces coated with a high-reflectivity dielectric mirror. Laser light enters through a small hole in one of the mirrors and is trapped in the material by reflection from the mirrors and by internal reflection from the other sides of the cylinder. The pump light is eventually absorbed by the working material which then fluoresces at higher energy. Ideally, the fluorescence escapes carrying heat from the working material. The object to be cooled is placed in thermal contact with the second of the mirrors, so that it is both shaded from the escaping fluorescent radiation and does not absorb the laser light.
Efficiency and power of an optical refrigerator are limited by radiation transfer effects. That is, some of the fluorescence radiation is reabsorbed by the cooling material, thereby changing the spectrum of the energy that ultimately escapes. Such reabsorption shifts the escaping fluorescent photons to lower energies, degrading the refrigerator performance. If the fluorescent quantum efficiency is sufficiently high, however, the solid will cool. R. I. Epstein et al. in Nature 377, 500 (1995) demonstrate that a solid may be optically pumped using monochromatic radiation such that the resulting fluorescence has an average photon energy higher than that of the pump radiation. This first experimental verification of cooling used a rectangular block of ytterbium-doped metal fluoride glass (Yb.sup.3+ -doped ZBLANP, a heavy metal fluoride glass containing zirconium, barium, lanthanum, aluminum, sodium and lead) and displayed a 2% cooling efficiency. Optical refrigeration can therefore be used to produce a practical optical refrigerator using currently available solid-state technology which would produce no vibrations and neither generate nor be affected by electromagnetic interference. It is estimated by the authors that, by using .sup.3+ -doped ZBLANP, this device would cool to .ltoreq.77 K from room temperature, convert .about.0.5% of the applied electric power to heat lift at 77 K, weigh less than 2 kg per watt of cooling power, and have many years of continuous operating lifetime.
The cooling efficiency of a fluorescent refrigerator is enhanced if the difference between the frequencies of pump radiation and the mean fluorescent radiation is increased. This also decreases the waste heat that has to be removed and allows optical refrigerators to operate at high powers. Conversely, if the fluorescence is shifted to lower frequencies, the cooling efficiency decreases.
Accordingly, it is an object of the present invention to shift the fluorescence spectrum of an optical refrigerator to higher energies, thereby improving refrigerator efficiency.
Another object of the present invention is to shift the fluorescence spectrum of an optical refrigerator by modifying the shape of the cooling element and by tuning the wavelength dependence of the reflectivity of dielectric mirrors employed for this purpose.
Yet another object of the invention is to convert the escaping fluorescence radiation to electrical power, thereby increasing the cooling efficiency.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.